This invention relates to telecommunications networks and in particular to the handling, suppression, of alarms generated in network elements, such as are used, for example, within the SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network Technologies) high-speed optical transmission technologies and other transmission systems.
SDH and SONET are high-speed transmission networks used, for example, in a telecommunications carrier's core network. The carrier may offer a diverse range of services which are continuously transmitted. The individual data is multiplexed into higher speeds and transmitted at speeds typically of 2.5 Gbps and above. Data is sent in time division multiplexed containers. Where no data is to be sent, an idle pattern is transmitted. There is a constant delay between each container. Full details of the SONET specification are set out in ANSI (American National Standards Institute) standard GR253 and of the SDH specification in the ITU-T (International Telecommunications Union) G709 Standard.
FIG. 1 shows a typical known network element comprising a pair of traffic cards 10, 12 shown as traffic card A and traffic card B. These cards pass data between themselves via a switch 14, which is connected to the traffic cards through traffic interfaces. Both the cards and the switch are connected to a controller card 16 via control interfaces and the controller card communicates with a remote management system. Examples of the network element shown may be found in many commercially available products including add-drop multiplexers sold under the trademarks SMA-1 Series 3, SMA-4 Series 3 and MSH 11c by Marconi Communications Limited of Coventry, England.
The traffic cards monitor themselves and also the traffic flow to determine and report fault conditions. Such fault conditions are signalled over the control interface to the controller card where they are processed and sent to the remote management system.
A single fault occurrence, particularly with regard to traffic, can cause alarms on many cards. Some of these will be as a direct consequence to others. Referring again to FIG. 1, network traffic arriving on traffic card A is passed to the switch 14, and routed to network card B to be sent out to the network. If the traffic source to card A fails, that card will detect the fault and generate an alarm signal. Traffic card B will also detect the fault and generate an alarm signal even though the fault is the failure of traffic to reach card A, which fault has already been detected and reported by card A. Thus, a single failure can cause more than one alarm to be generated with some of the alarms being generated by cards not directly affected by the fault. This leads to unnecessary alarm signals being reported to the controller and then to the management system. This is undesirable as it places an additional load on the communications interface to the management systems, imposes additional alarm processing requirements on the management system and additional complications to fault finding.
A known method of reducing the number of unnecessary alarms uses alarm consequential suppression. A hierarchical tree of alarms is used with lower level alarms being a consequence of higher level alarms. When a card processes alarms for onwards transmission, any lower level alarms are suppressed in the presence of a related higher level alarm. Suppression is dictated by the relationship contained in a suppression tree.
Such an arrangement works well on a single card. However, when functions are spread across a plurality of cards, for example, the two cards A and B in FIG. 1, the cards will act as single entities which have suppression trees that are exclusive of each other. They can, therefore, only suppress those alarms which are contained within themselves.
An alternative approach to alarm suppression correlates alarms across multiple cards and suppresses alarms at this level. This is possible as all alarms are collated by a control function and/or a management system. This correlation is preferably performed at the controller card that is local to each network element, to prevent multiple alarms being sent over the communications network. The solution has the disadvantage of being processor intensive.
There is, therefore, a need for an improved method and apparatus for handling alarm conditions in network elements which in part, at least, overcomes this problem.